Aha. I found an alternative means to import and export fdf data from a pdf form in Acrobat DC.
While in the document window, right click on the Toolbar, select "Customize Quick Tools," then, under the "Prepare Form" set of tools, select "More Form Options" and add it to the "Tools to show in Toolbar."
With that added to the Toolbar, then from a fielded pdf form you may access "More Form Options" from the additional tools dropdown menu ( ... ) on the Toolbar, where you will find Import Data and Export Data on the "More Form Options" submenu.